Reformer temperature control with leading temperature estimation

ABSTRACT

One concept relates to a method of controlling fuel reforming within an internal combustion engine exhaust line. Fuel injections are controlled using a predicted temperature, the predicted temperature being a temperature that would occur at some point in the future if predetermined assumptions are met. Preferably, the prediction is made using a model that includes terms for hydrocarbon storage and subsequent reaction within the reformer. The method improves reformer temperature control, particularly over periods during which the fuel supply to the reformer is pulsed. The scope of the invention also includes methods wherein a temperature is not specifically predicted, provided the control method takes into account hydrocarbon storage and subsequent reaction.

FIELD OF THE INVENTION

The present invention relates to pollution control systems and methodsfor diesel and lean burn gasoline engines.

BACKGROUND

NO_(x) emissions from diesel engines are an environmental problem.Several countries, including the United States, have long hadregulations pending that will limit NO_(x) emissions from trucks andother diesel-powered vehicles. Manufacturers and researchers have putconsiderable effort toward meeting those regulations.

In gasoline powered vehicles that use stoichiometric fuel-air mixtures,three-way catalysts have been shown to control NO_(x) emissions. Indiesel-powered vehicles, which use compression ignition, the exhaust isgenerally too oxygen-rich for three-way catalysts to be effective.

Several solutions have been proposed for controlling NOx emissions fromdiesel-powered vehicles. One set of approaches focuses on the engine.Techniques such as exhaust gas recirculation and partially homogenizingfuel-air mixtures are helpful, but these techniques alone will noteliminate NOx emissions. Another set of approaches remove NOx from thevehicle exhaust. These include the use of lean-burn NO_(x) catalysts,selective catalytic reduction (SCR), and lean NO_(x) traps (LNTs).

Lean-burn NOx catalysts promote the reduction of NO_(x) underoxygen-rich conditions. Reduction of NOx in an oxidizing atmosphere isdifficult. It has proven challenging to find a lean-burn NO_(x) catalystthat has the required activity, durability, and operating temperaturerange. Lean-burn NO_(x) catalysts also tend to be hydrothermallyunstable. A noticeable loss of activity occurs after relatively littleuse. Lean-burn NOx catalysts typically employ a zeolite wash coat, whichis thought to provide a reducing microenvironment. The introduction of areductant, such as diesel fuel, into the exhaust is generally requiredand introduces a fuel economy penalty of 3% or more. Currently, peak NOxconversion efficiencies for lean-burn NOx catalysts are unacceptablylow.

SCR generally refers to selective catalytic reduction of NOx by ammonia.The reaction takes place even in an oxidizing environment. The NOx canbe temporarily stored in an absorbent or ammonia can be fed continuouslyinto the exhaust. SCR can achieve high levels of NOx reduction, butthere is a disadvantage in the lack of infrastructure for distributingammonia or a suitable precursor. Another concern relates to the possiblerelease of ammonia into the environment.

LNTs are devices that adsorb NOx under lean exhaust conditions andreduce and release the adsorbed NOx under rich condition. A LNTgenerally includes a NOx absorbent and a catalyst. The absorbent istypically an alkaline earth oxide absorbent, such as BaCO₃ and thecatalyst is typically a precious metal, such as Pt or Ru. In leanexhaust, the catalyst speeds oxidizing reactions that lead to NOxadsorption. In a reducing environment, the catalyst activates reactionsby which adsorbed NOx is reduced and desorbed. In a typical operatingprotocol, a reducing environment will be created within the exhaust fromtime-to-time to regenerate (denitrate) the LNT.

A LNT can produce ammonia during denitration. Accordingly, it has beenproposed to combine a LNT and an ammonia SCR catalyst into one system.Ammonia produced by the LNT during regeneration is captured by the SCRcatalyst for subsequent use in reducing NOx, thereby improvingconversion efficiency over a stand-alone LNT with no increase in fuelpenalty or precious metal usage. U.S. Pat. No. 6,732,507 describes sucha system. U.S. Pat. Pub. No. 2004/0076565 describes such systems whereinboth components are contained within a single shell or disbursed overone substrate. WO 2004/090296 describes such a system wherein there isan inline reformer upstream of the LNT and the SCR catalyst.

Creating a reducing environment for LNT regeneration involveseliminating most of the oxygen from the exhaust and providing a reducingagent. Except where the engine can be run stoichiometric or rich, aportion of the reductant reacts within the exhaust to consume oxygen.The amount of oxygen to be removed by reaction with reductant can bereduced in various ways. If the engine is equipped with an intake airthrottle, the throttle can be used. The transmission gear ratio can bechanged to shift the engine to an operating point that produces equalpower but contains less oxygen. However, at least in the case of adiesel engine, it is generally necessary to eliminate some of the oxygenin the exhaust by combustion or reforming reactions with reductant thatis injected into the exhaust.

Reductant can be injected into the exhaust by the engine or a separatefuel injection device. For example, the engine can inject extra fuelinto the exhaust within one or more cylinders prior to expelling theexhaust. Alternatively, or in addition, reductant can be injected intothe exhaust downstream of the engine.

The reactions between reductant and oxygen can take place in the LNT,although it is generally preferred for the reactions to occur in acatalyst upstream of the LNT, whereby the heat of reaction does notcause large temperature increase within the LNT at every regeneration.

In addition to accumulating NOx, LNTs accumulate SOx. SOx is thecombustion product of sulfur present in ordinarily fuel. Even withreduced sulfur fuels, the amount of SOx produced by combustion issignificant. SOx adsorbs more strongly than NOx and necessitates a morestringent, though less frequent, regeneration. Desulfation requireselevated temperatures as well as a reducing atmosphere. The temperatureof the exhaust can be elevated by engine measures, particularly in thecase of a lean-burn gasoline engine, however, at least in the case of adiesel engine, it is often necessary to provide additional heat.Typically, this heat is provided through the same types of reactions asused to remove excess oxygen from the exhaust. The temperature of theLNT is generally controlled during desulfation, as the temperaturesrequired for desulfation are generally close to those at which the LNTcatalyst undergoes thermal deactivation.

U.S. Pat. No. 6,832,473 describes a system wherein the reductant isreformate produced outside the exhaust stream and injected into theexhaust as needed. During desulfations, the reformate is injectedupstream of an oxidation catalyst. Heat generated by combustion of thereformate over the oxidation catalyst is carried by the exhaust to theLNT and raises the LNT to desulfations temperatures.

U.S. Pat. Pub. No. 2003/0101713 describes an exhaust treatment systemwith a fuel reformer placed in the exhaust line upstream of a LNT. Thereformer includes both oxidation and reforming catalysts. The reformerboth removes excess oxygen and converts the diesel fuel reductant intomore reactive reformate. For desulfations, heat produced by the reformeris used to raise the LNT to desulfations temperatures.

U.S. Pat. Pub. No. 2003/0101713 describes a case in which endothermicreactions dominate and the reformer tends to cool when hydrocarbons areinjected at a rate that produces a desired concentration of reformate.Between pulses that produce reformate, fuel is injected at a reducedrate whereby exothermic reactions dominate and the reformer heats.

In spite of advances, there continues to be a long felt need for anaffordable and reliable exhaust treatment system that is durable, has amanageable operating cost (including fuel penalty), and is practical forreducing NOx emissions from diesel engines to a satisfactory extent inthe sense of meeting U.S. Environmental Protection Agency (EPA)regulations effective in 2010 and other such regulations.

SUMMARY

One of the inventor's concepts relates to a method of controlling fuelreforming within an internal combustion engine exhaust line. A reformeris supplied with fuel injected into the exhaust upstream of thereformer. The fuel injections are controlled using a predictedtemperature that is a temperature that would occur at some point in thefuture if predetermined assumptions are met. In general, the predictedtemperature is based in part on a temperature measurement. In apreferred embodiment, the prediction is made using a model that includesterms for hydrocarbon storage and subsequent reaction within thereformer. The method improves reformer temperature control, particularlyover periods where the fuel supply to the reformer is pulsed.

A further concept relates to a method of controlling a temperature of afuel reformer. The method comprises using a model to predict atemperature associated with the reformer and using the predictedtemperature in a temperature control algorithm. According to theconcept, the temperature prediction is made taking into account theeffects of hydrocarbon storage and subsequent reaction, which can resultin heating of the reformer following the termination of fuel injection.

A closely related concept is a method of controlling the temperature ofa fuel reformer comprising predicting a future reformer temperature. Thepredicted future temperature is used in a feedback control loop. Thepredictions take into account the effects of hydrocarbon storage andsubsequent reaction.

A further concept relates to a method of reforming within an internalcombustion engine exhaust line. Hydrocarbons are injected into theexhaust line upstream of a reformer. The amount of hydrocarbon adsorbedin the reformer is estimated and the reformer temperature is controlledbased in part on that estimate.

The primary purpose of this summary has been to present certain of theinventors' concepts in a simplified form to facilitate understanding ofthe more detailed description that follows. This summary is not acomprehensive description of every one of the inventor's concepts orevery combination of the inventor's concepts that can be considered“invention”. Other concepts of the inventor will be conveyed to one ofordinary skill in the art by the following detailed description togetherwith the drawings. The specifics disclosed herein may be generalized,narrowed, and combined in various ways with the ultimate statement ofwhat the inventor claim as his invention being reserved for the claimsthat follow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an exemplary exhaust treatmentsystem to which the inventor's concepts can be applied.

FIG. 2 is a schematic illustration of another exemplary exhausttreatment system to which the inventor's concepts can be applied.

FIG. 3 is a flow chart of a computational procedure conceived by theinventor.

FIG. 4 is a schematic illustration of control architecture in which someof the inventor's concepts can be applied.

FIG. 5 is a flow chart of a desulfation method in which some of theinventor's concepts can be applied.

DETAILED DESCRIPTION

FIG. 1 provides a schematic illustration of an exemplary powergeneration system 5 in which various concepts of the inventor can beimplemented. The system 5 comprises an engine 9, a transmission 8, andan exhaust aftertreatment system 7. The exhaust aftertreatment system 7includes a controller 10, a fuel injector 11, a lean NOx catalyst 15, areformer 12, a lean NOx-trap (LNT) 13, an ammonia-SCR catalyst 14, adiesel particulate filter (DPF) 16, and a clean-up catalyst 17. Thecontroller 10 receives data from several sources; including temperaturesensors 20 and 21 and NOx sensors 22 and 23. The controller 10 may be anengine control unit (ECU) that also controls the transmission 8 and theexhaust aftertreatment system 7 or may include several control unitsthat collectively perform these functions.

The exhaust from the engine 9 generally contains products of leancombustion including NOx, particulates, and some oxygen (typically5-15%). The lean-NOx catalyst 15 removes a portion of the NOx from theexhaust using reductants, typically hydrocarbons that form part of theexhaust or hydrocarbon that have been stored by the lean-NOx catalyst15. The DPF 16 removes particulates. During lean operation (a leanphase), the LNT 13 adsorbs a second portion of the NOx. The ammonia-SCRcatalyst 14 may have ammonia stored from a previous regeneration of theLNT 13 (a rich phase). If the ammonia-SCR catalyst 14 contains storedammonia, it removes a third portion of the NOx from the lean exhaust.The clean-up catalyst 17 may serve to oxidize CO and unburnedhydrocarbons.

FIG. 2 provides another exemplary system 25 in which various concepts ofthe inventor can be implemented. The system 25 contains many of the samecomponents as the system 5, although it does not include the lean NOxcatalyst 15 or the cleanup oxidation catalyst 17. Another difference isthat in the system 25 the DPF 16 is placed between the reformer 12 andthe LNT 13. In this configuration, the DPF 16 may serve to protect theLNT 13 from high temperatures during denitrations by providing a thermalbuffer between the reformer 12 and the LNT 13. Reducing the numberand/or magnitude of temperature excursions experienced by the LNT 13 mayextend its life.

From time-to-time, the LNT 13 must be regenerated to remove accumulatedNOx (denitrated). Denitration may involve first heating the reformer 12to an operational temperature by injecting fuel at a sub-stoichiometricrate with respect to the oxygen in the exhaust whereby the injected fuelreacts in the reformer 12 in an excess of oxygen. An operationaltemperature for the reformer 12 depends on the reformer design. Once thereformer 12 is sufficiently heated, denitration may proceed by injectingfuel at a super-stoichiometric rate whereby the reformer 12 consumesmost of the oxygen in the exhaust while producing reformate. Reformatethus produced reduces NOx adsorbed in the LNT 13. Some of this NOx isreduced to NH₃, most of which is captured by the ammonia-SCR catalyst 14and used to reduce NOx during a subsequent lean phase. The clean-upcatalyst 17 oxidizes unused reductants and unadsorbed NH₃ using storedoxygen or residual oxygen remaining in the exhaust during the richphases. During regeneration, the lean-NOx catalyst 15 may storereductant for later use.

From time-to-time, the LNT 13 must also be regenerated to removeaccumulated sulfur compounds (desulfated). Desulfation may involveheating the reformer 12 to an operational temperature, heating the LNT13 to a desulfating temperature, and providing the heated LNT 13 with areducing atmosphere. Desulfating temperatures vary, but are typically inthe range from about 500 to about 800° C., more typically in the rangefrom about 650 to about 750° C. Below a minimum temperature, desulfationis very slow. Above a maximum temperature, the LNT 13 may be damaged.

During these operations, the temperature of the reformer 12 is affectedby several factors. These factors may include, for example, exothermicreactions by which oxygen is consumed, endothermic reactions by whichreformate is produced, convective heat transfer into the reformer 12 bythe exhaust feeding the reformer 12, convective heat transfer out of thereformer 12 by exhaust exiting the reformer 12, and the thermal mass ofthe reformer 12.

In certain modes of operation, the balance of these factors has atendency to overheat the reformer 12. For example, during desulfationonly a small amount of reformate production may be desired and the heatreleased by exothermic reactions that remove excess oxygen from theexhaust may be far in excess of the heat taken up by endothermicreforming reactions and the heat taken up by the exhaust passing throughthe reformer 12. Also, during periods of high exhaust oxygenconcentration, the characteristics of the reformer 12 may be such thatthe reformer 12 cannot be operated efficiently, or not at all, at thehigh fueling rates required for auto-thermal reforming. In either ofthese cases where the reformer 12 has a tendency to overheat, periods offuel injection may need to be limited. In between periods of fuelinjection, the reformer 12 will cool. Once the reformer 12 has cooled toa sufficient degree, fuel injection can be resumed. This results inpulsed operation.

When the reformer 12 is on the verge of overheating, it may be possibleto cool the reformer 12 by increasing the fuel injection rate.Increasing the fuel injection rate will sometimes cool the reformer 12by increasing the ratio of endothermic to exothermic reactions. Such anoperation is within the scope of the inventor's concepts; however, insome situations increasing the fuel injection rate may be undesirable orineffective. For example, increasing the fuel injection rate may beundesirable if it produces more reformate than can be effectively usedby the exhaust aftertreatment system; increasing the fuel injection ratemay be undesirable if it would push the reformer 12 into a regime whereit does not operate effectively; and increasing the fuel injection ratemay be undesirable if it ultimately increases the likelihood ofoverheating due to the hydrocarbon adsorption phenomena discussedherein. Therefore, it is generally preferred that fuel injection bediscontinued when the reformer 12 is almost at a point where it willoverheat.

In order to determine when overheating is imminent, the inventorcontemplates using a model that takes into account hydrocarbon storage.A preferred model comprises a thermal model, which is a model based inpart on an energy conservation equation. A thermal model can be zero,one, two, or three-dimensional, although a zero-dimensional lumpedparameter model will generally suffice.

A lumped parameter model generally includes at least a term for heatconvection into the model system, a term for heat convection out of themodel system, a term for heat taken up by the reformer 12, and a termfor heat generated by chemical reaction. Heat losses to the surroundingcan also be considered, but generally have a small effect.

Preferably, the model tracks hydrocarbon storage within the reformer 12and eventual reaction of a portion of that stored hydrocarbon in thereformer 12. In one embodiment, hydrocarbon storage takes place when thereformer 12 is supplied with a rich feed and reaction of previouslystored hydrocarbon takes place when the reformer 12 is supplied with alean feed. By modeling hydrocarbon storage during rich phases andsubsequent reaction of a portion of that hydrocarbon during lean phases,the model predicts availability within the reformer 12 of a portion ofthe injected fuel in periods between temporally adjacent fuel pulses.Temporally adjacent pulses are two periods of continuous fuel injectionseparated in time by one period during which no fuel is injected.

The heat convection rate into the reformer 12 is the production of theexhaust specific heat, the exhaust temperature, and the exhaust massflow rate. The exhaust mass flow rate can be measured or estimated, forexample using an intake air flow rate measurements, an engine fuel flowrate measurement, or simply with data available from the engine controlunit (ECU). The temperature of the exhaust entering the reformer 12 canbe measured or determined from the operating point of the engine 9, forexample.

The heat convection rate out of the reformer 12 depends on thetemperature of the exhaust leaving the reformer 12. That temperature canbe measured. Where the temperature of the reformer 12 is measured, thereformer exhaust gas temperature can be approximated as equal to thereformer temperature.

The chemical reactions in the fuel reformer 12 can be modeled as acombination of the three following reactions:0.684 CH_(1.85)+O₂→0.684 CO₂+0.632 H₂O  (1)0.316 CH_(1.85)+0.316 H₂O→0.316 CO+0.608 H₂  (2)0.316 CO+0.316 H₂O→0.316 CO₂+0.316 H₂  (3)wherein CH_(1.85) represents an exemplary reductant, such as a dieselfuel, with a 1.85 ratio between carbon and hydrogen. Reaction (1) isexothermic complete combustion by which oxygen is consumed. Reaction (2)is endothermic steam reforming, which results in reformate production.Reaction (3) is the water-gas shift reaction.

In a preferred embodiment, Reaction (1) is assumed to proceed at a rateand to an extent dependent on temperature, oxygen availability when fuelis in excess, fuel availability when oxygen is in excess, butindependently of Reaction (2). The oxygen availability depends at leaston the oxygen flow rate into the reformer 12. If the reformer 12 has asignificant oxygen storage capacity, then it may be desirable toincludes terms for the rates of oxygen adsorption and desorption. Theoxygen concentration of the exhaust flowing into the reformer 12 canalso be measured or estimated using data available from the ECU. While asignificant temperature gradient generally exists within the reformer12, the purposes of the invention can generally be achieved using asingle average temperature for the reformer 12.

In the preferred embodiment, Reaction (2) is considered to proceed to anextent dependent on the availability of fuel after the effect ofReaction (1). Reaction (3) is considered third, and has the least impacton a thermal model. Reaction kinetics, adsorption rates, and desorptionrates depend on reactor geometry and composition and are best determinedexperimentally for a particular system.

FIG. 3 is a flow chart of an exemplary computational procedure 50 formodeling the temperature of the reformer 12. This procedure isimplemented in finite time steps. Two instances of the procedure may beused at one time. The first instance may track the actual process. Thesecond instance may be used to look forward in time and make predictionsto determine how much the temperature could increase if fuel injectionwere to cease and excess oxygen were to become available.

At the start of procedure 50, certain values are initialized. Thesetypically include the amount of stored hydrocarbon and the reformertemperature. After initialization, the process begins with operation 51.

Operation 51 accounts for Reaction (1). The reaction is treated asproceeding to the fullest extent of the available reagents, the extenttherefore being determined by the limiting reagent, whether that be fuelor oxygen. An amount of heat is added to the reformer 12 based on theextent of Reaction (1) and the size of the time step. The amounts offuel and oxygen for purposes of the following steps are reduced inaccordance with the extent of Reaction (1).

Operation 52 accounts for Reaction (2). Reaction (2) is assumed not toproceed at all if there is excess oxygen and all the fuel is consideredto have been consumed by Reaction (1). Where there is fuel remainingafter Reaction (1), then Reaction (2) proceeds to some extent. UnlikeReaction (1), the inventor has found that Reaction (2) should not beassumed to proceed to the extent of available reagents. Rather, Reaction(2) is preferably modeled with a limited efficiency. Typicalefficiencies may be from about 0.35 to about 0.7 based on thestoichiometry, with values in the range from about 0.45 to about 0.55having been used in the inventor's work. It is recognized, however, thatthe best values to use will depend on the particular reformer to whichthe model is applied and the system in which the reformer is used.Moreover, the efficiency depends on factors including, withoutlimitation, the reactor temperature and the exhaust flow rate, althoughthe inventor does not consider it generally necessary to take thesefactors into account. Heat is removed from the reformer 12 in accordancewith the extent of Reaction (2).

Operation 53 accounts for Reaction (3). In the preferred embodiment,Reaction (3) is assumed to proceed to equilibrium based on the exhaustcomposition following accounting for Reactions (1) and (2). Heat isadded to the reformer 12 in accordance with the extent of Reaction (3).

Operation 54 determines whether there is excess oxygen followingReactions (1)-(3). In general, there will be excess oxygen if thereformer is supplied with fuel at below the stoichiometric rate withrespect to the exhaust and there will not be excess oxygen if the fuelis supplied at a stoichiometric or higher rate. If there is excessoxygen, the process 50 proceeds with Operation 55. If there is not, theprocess 50 proceeds with Operation 58.

Taking the case of excess oxygen, Operation 55 accounts for the releaseof stored fuel. The release rate may be assumed to be a stoichiometricrate in proportion to the excess oxygen as long as stored fuel isavailable. Other assumptions may also be used, although thestoichiometric rate assumption is preferred.

Operation 56 accounts for the heat released by reaction of the releasedfuel. The extent of reaction may be assumed to be stoichiometric withrespect to the amount of excess oxygen. The term release is used in abroad sense: the fuel may react without physically moving from itsstored location. Operation 57 is a place-holder to account for slip ofreleased fuel. In the inventor's preferred embodiment, there is no fuelslip under conditions of excess oxygen.

Taking the case of excess fuel, Operation 58 determines the amount ofthe excess fuel that is stored. In one model, the fuel storage amount isa fraction of the fuel that is in liquid form. For example, it may beassumed that about 90% of the excess fuel is in liquid form and thatabout 45% of this liquid fuel becomes stored on the surfaces of thereformer 12. It should be understood that the inventor's concepts have alargely empirical basis and are independent of the actual mechanism offuel storage. The actual mechanism may be, for example, physicalabsorption or chemical adsorption. Whatever the actual mechanism, theinventor has found it can be sufficient to assume that the fuel storagerate is a fixed fraction of excess fuel flow rate. The remaining portionof the excess fuel that is not stored in the reformer 12 is consideredto slip from the reformer 12 and is tracked in Operation 59 for use inthe management of downstream devices.

Operation 60 adds or removes heat from the reformer 12 based onconvective heat transfer: the net heat added to or taken up by theexhaust passing through the reformer 12. Operation 61 advances a clockin preparation for the next iteration of the process 50. If the process50 is being applied to determine a peak predicted temperature of thereformer 12, the iterations may cease when the amount of stored fuel inthe reformer 12 is reduced to zero or when the temperature of thereformer 12 begins to decline.

FIG. 4 provides a schematic of an exemplary control architecture 100that can be used to control both the temperature of the reformer 12 andthe temperature of the LNT 13. The control architecture 100 includesinner and outer loop controls and uses a model of the reformer 12 thattracks hydrocarbon storage and subsequent release.

The LNT temperature controller 102 is activated by a desulfationscheduler/controller 101 that applies any appropriate criteria todetermine when to initiate a desulfation process. The LNT temperaturecontroller 102 considers a LNT temperature provided by a state estimator103. It is preferred to use an observer or state estimator to determinethe LNT temperature, because the LNT temperature responds comparativelyslowly to controllable parameters. If some form of prediction is notused, there is a risk of the LNT temperature exceeding an intendedlimit. An extrapolation based on the current measured temperature, itsrate of change, and an estimate of the temperature measurement delay isgenerally sufficient. However, a model of the LNT 13 can be used. Such amodel preferably takes into account reactions of hydrocarbons slippingfrom the reformer 12. These hydrocarbons can react with residual oxygenin the exhaust or with oxygen stored in the LNT 13. When there is nooxygen in the exhaust, some hydrocarbons may become stored in the LNT 13and subsequently react when oxygen becomes available. These processescan be modeled as they are for the reformer 12.

The output of the LNT temperature controller 102 is instructions for thereformer controller 106. The instructions may simply be instructions forthe reformer 12 to switch between active and inactive modes. During theactive mode, the reformer 12 is heated to a temperature suitable forreformate production and controlled to produce reformate subject to notoverheating the reformer 12. During an inactive mode, the reformer 12 isgenerally “off”, meaning there is no reductant injection and thereformer 12 is allowed to cool freely.

When the reformer 12 is to be active, the reformer controller 106regulates the reformer temperature at least by issuing commands to theinjection controller 107. The injection influences the state of thereformer 12, which is illustrated by block 108 in the exemplary controlarchitecture. A state includes all properties of the reformer, includingits temperature, the composition of the exhaust entering it, and thecomposition of the exhaust leaving it. The temperature portion of thereformer state 108 is estimated by the reformer temperature estimator105, and used to provide feedback for the reformer temperaturecontroller 106. Accordingly, steps 105-108 comprise the inner loop ofthe control process 100.

The reformer state 108 influences the LNT state 109. The temperatureportion of the LNT state 109 is temperature estimated by the LNTtemperature state estimator 103 to provide a temperature estimate thatis used by the LNT temperature control 102 in an outer control loop.

FIG. 5 illustrates a desulfation control process 200 consistent with thecontrol architecture 100. The process 200 begins with operation 201,determining whether desulfation is required. The determination may bemade in any suitable fashion. For example, desulfation may be scheduledperiodically, e.g., after every 30 hours of operation. Alternatively,the need for desulfation can be determined based on system performance,e.g., based on the activity of the LNT 13 following an extensivedenitration or based on the frequency with which denitration is requiredhaving increased to an excessive degree.

The desulfation process begins with operation 202, warming the reformer12. The reformer 12 can be heated in any suitable fashion. In thisexample, the reformer 12 is heated by injecting fuel at a rate thatkeeps the exhaust at or below a stoichiometric fuel to oxygen ratio.Substantially all the fuel thereby combusts in the reformer 12 toproduce heat with essentially no reformate production.

The LNT 13 heats while the reformer 12 is heating, however, after thereformer 12 is fully heated, the LNT 13 may still require furtherheating. If necessary, at or below stoichiometric operation may beextended to adequately heat the LNT 13. In one example, the LNT 13 isheated to a temperature of at least about 450° C. prior to commencingrich operation.

Once the warm-up phase is complete, operation 203 begins. The fuelinjection rate at this stage may be controlled to give a targetedreformate production rate. Where the controller 10 can throttle theengine air intake or select the transmission gear ratio, these controlparameters can be selected to facilitate the efficient production and/orusage of the reformate.

Operation 204 determines whether the reformer 12 is overheating.Preferably, this determination is made on the basis of a predictedtemperature wherein a predicted temperature is a temperature that willor could occur at a future time. In other words, a predicted temperatureis a temperature that would occur at some point in the future ifpredetermined assumptions are met. Predetermined means that theassumptions are made first and the temperature predicted second, basedon the assumptions. As used herein, the term prediction does not includean estimate of a current temperature based on past information, whichwill be referred to herein as an estimate to avoid confusion. Also, theterm “predicted future temperature” may be used to explicitlydistinguish an estimate of a current temperature. The main purpose ofusing a prediction herein is to account for the effect of hydrocarbonstorage and subsequent reaction. The prediction is therefore preferablymade on the basis of a model that takes into account this phenomenon.

A prediction of the type described herein is typically made using ameasured temperature and a predicted or possible temperature increase. Apossible temperature increase could be made on the basis of theassumption that the fuel dosing will stop in the next instant andthereafter an excess of oxygen will become available for combustion ofstored hydrocarbons. The model may look ahead over some finite intervalof time to determine the value at which the temperature will peak.

Some of the inventor's concepts can be implemented without attempting toaccurately predict a temperature. For example, a temperature at whichthe reformer 12 is determined to be on the verge of overheating can beset as a function that decreases with increasing hydrocarbon storageamount. As a more specific example, it can be assumed that the reformer12 will heat after fuel cut-off by an amount that is proportional theamount of stored fuel. The amount of heating can be used as the amountby which the limit temperature is reduced.

Another approach contemplated by the inventor is to make an effectiveprediction through the mechanics by which a temperature estimate isformed. For example, one method of forming a temperature estimate isKalman filtering. In Kalman filtering, a temperature estimate is made onthe basis of a blended average of a measured temperature and amodel-based estimate of the current temperature based on a past systemstate. The Kalman filter estimate can be converted to a prediction byusing artificial values to form the model-based estimate whereby themodel-based estimate is no longer intended to accurately estimate acurrent temperature. For example, the Kalman filter can be givenaccelerated dynamics. Accelerating the dynamics typically involvesreducing a term reflecting the heat capacity of the reformer. The modelprediction may also depart from an approximation of actual conditions byassuming the presence of excess oxygen not thought to be present undercurrent conditions.

When the reformer 12 is on the verge of overheating, operation 205 shutsoff the fuel injection. In operation 206, the process 200 waits whilethe reformer 12 cools. The length of the waiting period can bedetermined in any suitable fashion. In one example, operation 206 lastsuntil the reformer 12 has cooled to a target temperature. In anotherexample, there is a fixed period between each fuel pulse. In a furtherexample, the length of the period is selected dynamically by thecontroller as part of a process of optimizing the amount of reformateproduction per unit fuel injected.

Operation 207 determines whether the LNT 13 is getting too hot. Atemperature prediction is preferably used in making this determinationas the LNT 13 may heat considerably following the termination of fuelinjection. If the LNT is getting too hot, operation 208 terminates thefuel injection. Terminating the fuel injection may comprise issuinginstructions to the inner loop control. If the LNT is not getting toohot, the process continues with Operation 210, which checks whetherdesulfation is complete. Fuel injection continues in Operation 203 iffuel injection is not complete and terminates in Operation 211 ifdesulfation is complete.

Operation 209 is another waiting operation. In one example, thiscomprises waiting until the LNT 13 has cooled to a target temperature.Preferably, however, there is a fixed period between phases of activefuel injection on the longer time scale determined by the outer loopcontrols.

Following operation 209, the reformer 12 is heated again in operation202. If the reformer 12 is of the type that must be heated to operateeffectively, heating is generally necessary following a period of nofuel injection during which the LNT 13 is allowed to cool. The periodsof no fuel injection to cool the reformer 12 are generally shorter andare normally selected to avoid having to reheat the reformer 12 to atemperature suitable for producing reformate. After the longer periodsof no fuel injection to cool the LNT 13, the reformer 12 is generallytoo cool to effectively produce reformate without a heating period. Sucha heating period generally comprises fuel injection at asub-stoichiometric rate with respect to the exhaust oxygen content.

While the engine 9 is preferably a compression ignition diesel engine,the various concepts of the invention are applicable to power generationsystems with lean-burn gasoline engines or any other type of engine thatproduces an oxygen rich, NOx-containing exhaust. For purposes of thepresent disclosure, NOx consists of NO and NO₂.

The transmission 8 can be any suitable type of automatic transmission.The transmission 8 can be a conventional transmission such as acounter-shaft type mechanical transmission, but is preferably a CVT. ACVT can provide a much larger selection of operating points than aconventional transmission and generally also provides a broader range oftorque multipliers. In general, a CVT will also avoid or minimizeinterruptions in power transmission during shifting. Examples of CVTsystems include hydrostatic transmissions; rolling contact tractiondrives; overrunning clutch designs; electrics; multispeed gear boxeswith slipping clutches; and V-belt traction drives. A CVT may involvepower splitting and may also include a multi-step transmission.

A preferred CVT provides a wide range of torque multiplication ratios,reduces the need for shifting in comparison to a conventionaltransmission, and subjects the CVT to only a fraction of the peak torquelevels produced by the engine. This can be achieved using a step-downgear set to reduce the torque passing through the CVT. Torque from theCVT passes through a step-up gear set that restores the torque. The CVTis further protected by splitting the torque from the engine, andrecombining the torque in a planetary gear set. The planetary gear setmixes or combines a direct torque element transmitted from the enginethrough a stepped automatic transmission with a torque element from aCVT, such as a band-type CVT. The combination provides an overall CVT inwhich only a portion of the torque passes through the band-type CVT.

The fuel injector 11 can be of any suitable type. Preferably, itprovides the fuel in an atomized or vaporized spray. The fuel may beinjected at the pressure provided by a fuel pump for the engine 9.Preferably, however, the fuel passes through a pressure intensifieroperating on hydraulic principles to at least double the fuel pressurefrom that provided by the fuel pump to provide the fuel at a pressure ofat least about 4 bar.

The lean-NOx catalyst 15 can be either an HC-SCR catalyst, a CO-SCRcatalyst, or a H₂-SCR catalyst. Examples of HC-SCR catalysts includetransitional metals loaded on refractory oxides or exchanged intozeolites. Examples of transition metals include copper, chromium, iron,cobalt, nickel, cadmium, silver, gold, iridium, platinum and manganese,and mixtures thereof. Exemplary of refractory oxides include alumina,zirconia, silica-alumina, and titania. Useful zeolites include ZSM-5, Yzeolites, Mordenite, and Ferrerite. Preferred zeolites have Si:Al ratiosgreater than about 5, optionally greater than about 20. Specificexamples of zeolite-based HC-SCR catalysts include Cu-ZSM-5, Fe-ZSM-5,and Co-ZSM-5. A CeO₂ coating may reduce water and SO₂ deactivation ofthese catalysts. Cu/ZSM-5 is effective in the temperature range fromabout 300 to about 450° C. Specific examples of refractory oxide-basedcatalysts include alumina-supported silver. Two or more catalysts can becombined to extend the effective temperature window.

Where a hydrocarbon-storing function is desired, zeolites can beeffective. U.S. Pat. No. 6,202,407 describes HC-SCR catalysts that havea hydrocarbon storing function. The catalysts are amphoteric metaloxides. The metal oxides are amphoteric in the sense of showingreactivity with both acids and bases. Specific examples includegamma-alumina, Ga₂O₃, and ZrO₂. Precious metals are optional. Whereprecious metals are used, the less expensive precious metals such as Cu,Ni, or Sn can be used instead of Pt, Pd, or Rh.

In the present disclosure, the term hydrocarbon is inclusive of allspecies consisting essentially of hydrogen and carbon atoms, however, aHC-SCR catalyst does not need to show activity with respect to everyhydrocarbon molecule. For example, some HC-SCR catalysts will be betteradapted to utilizing short-chain hydrocarbons and HC-SCR catalysts ingeneral are not expected to show substantial activity with respect toCH₄.

Examples of CO-SCR catalysts include precious metals on refractory oxidesupports. Specific examples include Rh on a CeO₂—ZrO₂ support and Cuand/or Fe ZrO₂ support.

Examples of H₂-SCR catalysts also include precious metals on refractoryoxide supports. Specific examples include Pt supported on mixed LaMnO₃,CeO₂, and MnO₂, Pt supported on mixed ZiO₂ and TiO₂, Ru supported onMgO, and Ru supported on Al₂O₃.

The lean-NOx catalyst 15 can be positioned differently from illustratedin FIG. 1. In one embodiment, the lean NOx catalyst 15 is upstream ofthe fuel injector 11. In another embodiment the lean NOx catalyst isdownstream of the reformer 12, whereby the lean NOx catalyst 15 can usereformer products as reductants. In a further embodiment, the lean NOxcatalyst 15 is well downstream of the LNT 13, whereby the lean NOxcatalyst 15 can be protected from high temperatures associated withdesulfating the LNT 13.

A fuel reformer is a device that converts heavier fuels into lightercompounds without fully combusting the fuel. A fuel reformer can be acatalytic reformer or a plasma reformer. Preferably, the reformer 12 isa partial oxidation catalytic reformer. A partial oxidation catalyticreformer comprises a reformer catalyst. Examples of reformer catalystsinclude precious metals, such as Pt, Pd, or Ru, and oxides of Al, Mg,and Ni, the later group being typically combined with one or more ofCaO, K₂O, and a rare earth metal such as Ce to increase activity. Areformer is preferably small in size as compared to an oxidationcatalyst or a three-way catalyst designed to perform its primaryfunctions at temperatures below 500° C. A partial oxidation catalyticreformer is generally operative at temperatures from about 600 to about1100° C. A preferred reformer has a low thermal mass and a low catalystloading as compared to a device designed to produce reformate at exhaustgas temperatures.

The NOx absorber-catalyst 13 can comprise any suitable NOx-adsorbingmaterial. Examples of NOx adsorbing materials include oxides,carbonates, and hydroxides of alkaline earth metals such as Mg, Ca, Sr,and Be or alkali metals such as K or Ce. Further examples ofNOx-adsorbing materials include molecular sieves, such as zeolites,alumina, silica, and activated carbon. Still further examples includemetal phosphates, such as phosphates of titanium and zirconium.Generally, the NOx-adsorbing material is an alkaline earth oxide. Theabsorbent is typically combined with a binder and either formed into aself-supporting structure or applied as a coating over an inertsubstrate.

The LNT 13 also comprises a catalyst for the reduction of NOx in areducing environment. The catalyst can be, for example, one or moreprecious metals, such as Au, Ag, and Cu, group VIII metals, such as Pt,Pd, Ru, Ni, and Co, Cr, Mo, or K. A typical catalyst includes Pt and Rh,although it may be desirable to reduce or eliminate the Rh to favor theproduction of NH₃ over N₂. Precious metal catalysts also facilitate theabsorbent function of alkaline earth oxide absorbers.

Absorbents and catalysts according to the present invention aregenerally adapted for use in vehicle exhaust systems. Vehicle exhaustsystems create restriction on weight, dimensions, and durability. Forexample, a NOx absorbent bed for a vehicle exhaust systems must bereasonably resistant to degradation under the vibrations encounteredduring vehicle operation.

An absorbent bed or catalyst brick can have any suitable structure.Examples of suitable structures may include monoliths, packed beds, andlayered screening. A packed bed is preferably formed into a cohesivemass by sintering the particles or adhering them with a binder. When thebed has an absorbent function, preferably any thick walls, largeparticles, or thick coatings have a macro-porous structure facilitatingaccess to micro-pores where adsorption occurs. A macro-porous structurecan be developed by forming the walls, particles, or coatings from smallparticles of adsorbant sintered together or held together with a binder.

The ammonia-SCR catalyst 14 is a catalyst effective to catalyzereactions between NOx and NH₃ to reduce NOx to N₂ in lean exhaust.Examples of SCR catalysts include oxides of metals such as Cu, Zn, V,Cr, Al, Ti, Mn, Co, Fe, Ni, Pd, Pt, Rh, Rd, Mo, W, and Ce, zeolites,such as ZSM-5 or ZSM-11, substituted with metal ions such as cations ofCu, Co, Ag, Zn, or Pt, and activated carbon. Preferably, the ammonia-SCRcatalyst 14 is designed to tolerate temperatures required to desulfatethe LNT 13.

The particulate filter 16 can have any suitable structure. Examples ofsuitable structures include monolithic wall flow filters, which aretypically made from ceramics, especially cordierite or SiC, blocks ofceramic foams, monolith-like structures of porous sintered metals ormetal-foams, and wound, knit, or braided structures of temperatureresistant fibers, such as ceramic or metallic fibers. Typical pore sizesfor the filter elements are about 10 μm or less. Optionally, one or moreof the reformer 12, the LNT 13, the lean-NOx catalyst 15, or theammonia-SCR catalyst 14 is integrated as a coating or within thestructure of the DPF 16.

The DPF 16 is regenerated to remove accumulated soot. The DPF 16 can beof the type that is regenerated continuously or intermittently. Forintermittent regeneration, the DPF 16 is heated, using a reformer 12 forexample. The DPF 16 is heated to a temperature at which accumulated sootcombusts with O₂. This temperature can be lowered by providing the DPF16 with a suitable catalyst. After the DPF 16 is heated, soot iscombusted in an oxygen rich environment.

For continuous regeneration, the DPF 16 may be provided with a catalystthat promotes combustion of soot by both NO₂ and O₂. Examples ofcatalysts that promote the oxidation of soot by both NO₂ and O₂ includeoxides of Ce, Zr, La, Y, and Nd. To completely eliminate the need forintermittent regeneration, it may be necessary to provide an additionaloxidation catalyst to promote the oxidation of NO to NO₂ and therebyprovide sufficient NO₂ to combust soot as quickly as it accumulates.Where regeneration is continuous, the DPF 16 is suitably placed upstreamof the reformer 12. Where the DPF 16 is not continuously regenerated, itis generally positioned as illustrated downstream of the reformer 12. Anadvantage of the position illustrated in FIG. 2 is that the DPF 16buffers the temperature between the reformer 12 and the LNT 13.

The clean-up catalyst 17 is preferably functional to oxidize unburnedhydrocarbons from the engine 9, unused reductants, and any H₂S releasedfrom the NOx absorber-catalyst 13 and not oxidized by the ammonia-SCRcatalyst 15. Any suitable oxidation catalyst can be used. A typicaloxidation catalyst is a precious metal, such as platinum. To allow theclean-up catalyst 17 to function under rich conditions, the catalyst mayinclude an oxygen-storing component, such as ceria. Removal of H₂S,where required, may be facilitated by one or more additional componentssuch as NiO, Fe₂O₃, MnO₂, CoO, and CrO₂.

The invention as delineated by the following claims has been shownand/or described in terms of certain concepts, components, and features.While a particular component or feature may have been disclosed hereinwith respect to only one of several concepts or examples or in bothbroad and narrow terms, the components or features in their broad ornarrow conceptions may be combined with one or more other components orfeatures in their broad or narrow conceptions wherein such a combinationwould be recognized as logical by one of ordinary skill in the art.Also, this one specification may describe more than one invention andthe following claims do not necessarily encompass every concept, aspect,embodiment, or example described herein.

1. A method of fuel reforming within an internal combustion engineexhaust line, comprising: injecting fuel into the exhaust line upstreamof a fuel reformer; measuring a temperature within the exhaust line;predicting a temperature based in part on the measured temperature; andcontrolling the fuel injection using the predicted temperature; whereinthe predicted temperature is a temperature that would occur at somepoint in the future if predetermined assumptions are met.
 2. The methodof claim 1, wherein controlling the fuel injection using the predictedtemperature comprises temporarily discontinuing the fuel injection ifthe predicted temperature meets or exceeds a critical value.
 3. Themethod of claim 1, wherein injecting fuel into the exhaust line upstreamof the fuel reformer comprises: injecting fuel at rate that produces asub-stoichiometric concentration of fuel in the exhaust line to heat thefuel reformer to a temperature suitable for producing reformate; andsubsequently injecting fuel at a higher rate to produce asuper-stoichiometric concentration of fuel in the exhaust line in orderto produce reformate.
 4. The method of claim 1, wherein the temperatureprediction is based in part on a model.
 5. The method of claim 4,wherein the fuel is injected in pulses and the model predicts theavailability within the reformer of a portion of the injected fuel inperiods between temporally adjacent fuel pulses.
 6. The method of claim4, wherein the model takes into account fuel storage within the fuelreformer and subsequent reaction of the stored fuel.
 7. The method ofclaim 4, wherein the model predicts the reformer will heat in periodsfollowing the termination of fuel injection due to reactions ofpreviously injected fuel within the reformer.
 8. The method of claim 4,wherein the model predicts that some of the injected fuel will absorbwithin the fuel reformer without immediately reacting, but willsubsequently react.
 9. An exhaust treatment system comprising acontroller, wherein the controller implements the method of claim
 1. 10.A vehicle comprising the exhaust treatment system, of claim
 9. 11. Amethod of controlling the temperature of a fuel reformer, comprising:using a model to predict a temperature associated with the reformer; andusing the predicted temperature in a temperature control algorithm;wherein the temperature prediction takes into account hydrocarbonstorage and subsequent reaction.
 12. The method of claim 11, wherein thefuel reformer is configured within an exhaust line upstream of a leanNOx trap.
 13. The method of claim 12, wherein fuel is injected into theexhaust line in pulses during a regeneration of the lean NOx trap. 14.The method of claim 11, wherein the model predicts the reformertemperature using dynamics that are faster than the actual dynamics areexpected to be.
 15. A method of controlling the temperature of a fuelreformer, comprising: predicting future reformer temperatures; and usingthe predicted future reformer temperatures in a feedback control loop;wherein the predictions take into account hydrocarbon storage andsubsequent availability for reaction.
 16. The method of claim 15,wherein the fuel reformer is configured within an exhaust line upstreamof a lean NOx trap.
 17. The method of claim 15, wherein fuel is injectedinto the exhaust line in pulses during a regeneration of the lean NOxtrap.
 18. A method of reforming within an internal combustion engineexhaust line, comprising: injecting hydrocarbons into the exhaust lineupstream of a reformer; estimating hydrocarbon storage by the reformer;controlling the reformer temperature based in part on the hydrocarbonstorage estimate.
 19. The method of claim 18, wherein the fuel reformeris configured within an exhaust line upstream of a lean NOx trap. 20.The method of claim 18, wherein fuel is injected into the exhaust linein pulses during a regeneration of the lean NOx trap.
 21. The method ofclaim 18, wherein the hydrocarbon storage estimate is used in a thermalmodel of the reformer.
 22. The method of claim 21, wherein the model isapplied with accelerated dynamics.
 23. The method of claim 18,controlling the reformer temperature based in part on the hydrocarbonstorage estimate comprises temporarily terminating the hydrocarboninjection if the estimated amount of hydrocarbon stored is too high. 24.The method of claim 18, wherein injecting hydrocarbons into the exhaustline upstream of a reformer comprises: injecting hydrocarbons at ratethat produces a sub-stoichiometric concentration of hydrocarbons in theexhaust line to heat the fuel reformer to a temperature suitable forproducing reformate; and subsequently injecting hydrocarbons at a higherrate to produce a super-stoichiometric concentration of fuel in theexhaust line in order to produce reformate.